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The present invention relates generally to integrated angular rate and acceleration sensors (xe2x80x9cmulti-sensorsxe2x80x9d), and more specifically to a micro-machined multi-sensor capable of providing 1-axis of acceleration sensing and 2-axes of angular rate sensing, and a technique of fabricating such a multi-sensor.
Micro-machined multi-sensors are known that comprise at least one accelerometer for providing indications of acceleration sensing and angular rate sensing in a single sensor device. A conventional micro-machined multi-sensor, as described in U.S. Pat. No. 5,392,650 issued Feb. 28, 1995 entitled MICRO-MACHINED ACCELEROMETER GYROSCOPE, comprises a pair of accelerometers, in which each accelerometer includes a rigid accelerometer frame anchored to a substrate, and a proof mass suspended from the rigid frame by a plurality of flexures. The micro-machined multi-sensor typically has a single acceleration-sensing axis and a single rotation-sensing axis perpendicular to the acceleration axis associated therewith. Further, the micro-machined multi-sensor is typically configured for simultaneously vibrating the proof masses in antiphase along a vibration axis, which is perpendicular to the acceleration and rotation axes.
In the event the conventional micro-machined multi-sensor is subjected to linear and rotational motions while the proof masses are simultaneously vibrated in an antiphase manner, forces of linear and Coriolis acceleration are generated that deflect the proof masses relative to the substrate. The multi-sensor is configured to sense the deflections of the respective proof masses, and to produce corresponding acceleration sense signals having values proportional to the magnitude of the deflection. Because the responses of the vibrating proof masses to linear acceleration are in phase and the responses of the proof masses to Coriolis acceleration are in antiphase, the linear acceleration components (containing the acceleration sensing information) and the rotational acceleration components (containing the angular rate sensing information) of the sense signals can be separated by adding or subtracting the signals to cancel the rotational or linear components, respectively.
One drawback of the above-described conventional micro-machined multi-sensor is that it typically provides only 1-axis of acceleration sensing and only 1-axis of angular rate sensing. However, it is often advantageous to provide more than one axis of acceleration sensing and/or angular rate sensing in a single sensor device.
A second conventional micro-machined sensor capable of measuring rates of rotation relative to two rotation-sensing axes is described in U.S. Pat. No. 5,869,760 issued Feb. 9, 1999 entitled MICRO-MACHINED DEVICE WITH ROTATIONALLY VIBRATED MASSES. The micro-machined sensor comprises a pair of accelerometers, in which each accelerometer includes a mass in the form of a circular beam suspended over a substrate by a plurality of flexures, and an adjacent pair of acceleration-sensing electrodes. The two rotation-sensing axes associated with the micro-machine sensor are in the plane of the substrate. Further, the micro-machined sensor is configured for rotationally vibrating the circular beams in an antiphase manner, i.e., alternately rotating one circular beam clockwise/counterclockwise while the other beam simultaneously rotates in the opposite direction by substantially the same amount.
In the event the second conventional micro-machined sensor is subjected to linear and rotational motions while the circular beams are simultaneously rotated in antiphase, forces of linear and Coriolis acceleration are generated that deflect the beams relative to the substrate. The acceleration-sensing electrodes sense the deflections of the respective beams, and produce corresponding acceleration sense signals proportional to the magnitude of the deflection and the rate of rotation relative to the rotation-sensing axes. Because the sign of the rotational acceleration components (containing the angular rate sensing information) of the sense signals corresponds to the direction of rotation of the circular beams, the rotational components can be separated from the linear acceleration components of the sense signals by subtracting the signals to cancel the linear components. However, although the micro-machined sensor is capable of providing more than one axis of angular rate sensing, it has drawbacks in that it typically provides no acceleration sensing information.
Conventional techniques of fabricating micro-machined sensors and multi-sensors are known that employ layers of sacrificial and structural material in the fabrication process. One such fabrication technique is known as surface micro-machining, in which a micro-machined device is fabricated substantially onto the surface of a substrate. The conventional surface micro-machining technique includes depositing a layer of sacrificial material (e.g., silicon dioxide, SiO2) or structural material (e.g., polysilicon) onto the surface of the substrate (e.g., silicon). The structural material is employed in the construction of functional components of the micro-machined device, and the sacrificial material is subsequently removed in a final step of the fabrication process. The deposited layer of sacrificial or structural material is masked with a mask pattern, which is typically transferred using a photolithographic process. Next, the underlying material not protected by the mask is etched to transfer the mask pattern to that particular material layer. The depositing, masking, and etching steps are then repeated until the construction of the functional components of the micro-machined device is completed. Finally, one or more portions of the structural material are released by etching or otherwise removing the underlying and/or surrounding sacrificial material. The conventional surface micro-machining fabrication technique is typically low-cost and generally permits electronic circuitry to be incorporated near the functional components of the micro-machined device.
However, the conventional surface micro-machining technique has drawbacks when employed in fabricating micro-machined sensors and multi-sensors. For example, a micro-machined multi-sensor typically includes at least one functional component whose alignment and/or width are critical to the optimal performance of the sensor device. Because the mask patterns used in the construction of the functional components of the sensor device are typically laid out according to the horizontal and vertical spacings of a rectilinear grid, it can be difficult to obtain such critical alignments and widths of the functional sensor components.
It would therefore be desirable to have a micro-machined multi-sensor device that provides both acceleration sensing and angular rate sensing, and avoids the drawbacks of the above-described conventional micro-machined sensor devices.
In accordance with the present invention, a micro-machined multi-sensor is disclosed that provides 1-axis of acceleration sensing and 2-axes of angular rate sensing. The presently disclosed micro-machined multi-sensor comprises at least one pair of accelerometers, which provide electrically independent acceleration sense signals including information pertaining to acceleration sensing and angular rate sensing relative to one or more sensing axes.
In a first embodiment, the micro-machined multi-sensor comprises a pair of accelerometers, each accelerometer including a mass suspended over and anchored to a substrate by a plurality of flexures. The multi-sensor has two associated mutually orthogonal rotation-sensing axes in the plane of the substrate, and one associated acceleration-sensing axis perpendicular to the two rotation axes. Further, each mass has lateral and longitudinal axes of symmetry and a driven rotation axis perpendicular to the lateral and longitudinal axes associated therewith. Each accelerometer further includes a first pair of acceleration sense electrode structures disposed along the lateral axis, and a second pair of acceleration sense electrode structures disposed along the longitudinal axis of the respective masses. The multi-sensor further comprises a fork member configured to couple the two masses to allow relative antiphase movement and to resist in phase movement of the masses. The pluralities of flexures anchoring the masses to the substrate are configured to constrain the masses to move substantially only in a rotational manner relative to the substrate.
In the presently disclosed embodiment, the micro-machined multi-sensor comprises a drive electrode structure configured for rotationally vibrating the masses in antiphase, i.e., alternately rotating one mass clockwise/counterclockwise about its rotation axis, while the other mass simultaneously rotates about its rotation axis in the opposite direction by substantially the same amount. In the event the multi-sensor with the rotationally vibrating masses is subjected to linear and/or rotational motion, the first and second pairs of acceleration sense electrodes produce electrically independent acceleration sense signals based on forces of linear and Coriolis acceleration imposed on the masses. The multi-sensor is configured (1) to add the difference of the accelerations sensed by the first pair of acceleration sense electrodes of the first accelerometer, and the difference of the accelerations sensed by the first pair of acceleration sense electrodes of the second accelerometer, to obtain information pertaining to angular rate sensing relative to the lateral rotation axis of the multi-sensor, (2) to add the difference of the accelerations sensed by the second pair of acceleration sense electrodes of the first accelerometer, and the difference of the accelerations sensed by the second pair of acceleration sense electrodes of the second accelerometer, to obtain information pertaining to angular rate sensing relative to the longitudinal rotation axis of the multi-sensor, and (3) to add the sum of the accelerations sensed by the first pair of acceleration sense electrodes of the first accelerometer, the sum of accelerations sensed by the first pair of acceleration sense electrodes of the second accelerometer, the sum of accelerations sensed by the second pair of acceleration sense electrodes of the first accelerometer, and the sum of accelerations sensed by the second pair of acceleration sense electrodes of the second accelerometer, to obtain information pertaining to acceleration sensing relative to the acceleration axis of the multi-sensor.
In a second embodiment, the micro-machined multi-sensor comprises two pairs of accelerometers arranged to form a square. Each accelerometer includes a mass suspended over and anchored to a substrate. The multi-sensor further comprises respective fork members coupling adjacent pairs of masses to allow relative antiphase movement and to resist in phase movement of the adjacent masses. The micro-machined multi-sensor has two associated, mutually orthogonal rotation-sensing axes in the plane of the substrate, and one associated acceleration-sensing axis perpendicular to the two rotation axes. Each accelerometer further includes a first pair of acceleration sense electrode structures disposed along a lateral axis, and a second pair of acceleration sense electrode structures disposed along a longitudinal axis of the respective mass. The two pairs of accelerometers are arranged in mirror image fashion on opposite sides of the respective rotation axes. Because of the enhanced symmetry of this second embodiment of the micro-machined multi-sensor, the multi-sensor device can be more easily centered on a die, thereby reducing adverse effects of die surface area distortions and gradients.
In a third embodiment, a method of fabricating the micro-machined multi-sensor includes depositing a layer of sacrificial material or structural material onto the substrate surface. The structural material is employed in the construction of functional components of the sensor device, and the sacrificial material is subsequently removed in a final step of the fabrication method. The deposited layer of sacrificial or structural material is then masked with a predetermined mask pattern, which is formed using a rectilinear grid having multiple horizontal and vertical spacings. The mask pattern is employed to define the functional components of the sensor device. In the event the micro-machined multi-sensor includes at least one first functional component whose alignment and/or width are critical to the optimal performance of the sensor device, the first functional component is defined by the mask pattern so that its longitudinal axis is substantially parallel to the horizontal or vertical axis of the mask. In the event the micro-machined multi-sensor includes at least one second functional component whose alignment and/or width are not critical to optimal sensor performance, the second functional component may be defined by the mask pattern so that its longitudinal axis is not parallel to the horizontal and vertical axes of the mask.
By configuring the above-described micro-machined multi-sensors to include at least one pair of accelerometers, each accelerometer having a mass and providing two pairs of electrically independent acceleration sense signals along lateral and longitudinal axes of symmetry of the mass, respectively, 1-axis of acceleration sensing and 2-axes of angular rate sensing can be obtained by suitably adding and/or subtracting the acceleration sense signals. Further, by defining functional components of the sensor device with at least one mask so that components having critical alignments and/or physical dimensions are disposed substantially parallel to the horizontal or vertical axis of the mask, and non-critical components may be oriented off the mask axes, improved sensor performance can be achieved.
Other features, functions, and aspects of the invention will be evident from the Detailed Description of the Invention that follows.